[单项选择]If only I （） hard in the past few months! But I didn’t work hard enough and failed the maths examination.
A. work
B. worked
C. have worked
D. had worked

[单项选择]In the past few decades, remarkable findings have been made in ethnology, the study of animal social behavior. Earlier scientists had (61) that nonhuman social life was almost totally instinctive or fixed by genetics. Much more careful observation has shown that (62) variation occurs among the social ties of most species, showing that learning is a part of social life. That is, the (63) are not solely fixed by the genes. (64) , the learning that occurs is often at an early age in a process that is called imprinting. Imprinting is clearly (65) instinctive, but it is not quite like the learning of humans; it is something in between the two. An illustration best (66) the nature of imprinting. Once, biologists thought that ducklings followed the mother duck because of instincts. Now we know that, shortly (67) they hatch, ducklings fix (68) any object about the size of a duck and will henceforth follow it. So ducklings may follow a ba
A. assumed
B. adopted
C. believed
D. surmised

[单项选择]The growth of population during the past few centuries is no proof that population will continue to grow straight upward toward infinity and doom. On the contrary, demographic history offers evidence that population growth has not been at all constant. According to paleo-ecologist Edward Deevey, the past million years show three momentous changes. The first, a rapid increase in population around one million B, C, followed the innovations of tool-making and tool-using. But when the new power from the use of tools had been exploited, the rate of world population growth fell and became almost stable.
The next rapid jump in population started perhaps 10,000 years ago, when men began to keep herds, plow and plant the earth. Once again when initial productivity gains had been absorbed, the rate of population growth abated.
These two episodes suggest that the third great change, the present rapid growth, which began in the West between 250 and 350 years ago, may also slow down
A. One.
B. Two.
C. Three.
D. Non